This invention relates to both a system and a method for converting an AGTAW welder into an AGMAW welder.
Many companies having a need for performing welding operations have invested in automatic tungsten inert gas welders known in the art as AGTAW welding systems. However, as will be set forth in more detail hereinafter, such AGTAW welders are limited in the amount of metal they can deposit, and hence are too slow to effectively implement certain kinds of welds requiring large amounts of metal deposition. AGMAW (automatic gas metal arc) welding systems are also known in the prior art that are capable of depositing metal at rates an order of magnitude faster than AGTAW welders. Unfortunately, it is frequently not cost-effective for a company to own both an AGTAW and an AGMAW system as there is a much less frequent need for the high deposition welds that the latter can implement. Accordingly, the principal purpose of this invention is to provide a method for easily and temporarily converting an AGTAW welder into a AGMAW welder. But to fully appreciate the utility of the invention, some background as to the structure of AGTAW and AGMAW welders is necessary.
AGTAW welders typically comprise a weldhead including a tungsten electrode which is automatically movable around a workpiece by means of one or more servomotors, a power supply for supplying up to 300 amps of current to the tungsten electrode in the weldhead, a water cooling system for circulating water through the weldhead in order to cool it during a welding operation, and a control circuit assembly including a programmer for controlling the electrical characteristics of the output of the power supply connected to the tungsten electrode. Such AGTAW welders further include a source of inert gas connected to the weldhead for displacing corrosive atmospheric oxygen away from the molten puddle created during a welding operation, as well as a feeding mechanism for feeding weld wire to the working end of the tungsten electrode. The wire feeding mechanism in turn includes a motor for driving the wire at a selected speed (which may be as high as 100 inches per minute), and a motor control circuit which receives commands from the control circuit assembly of the welder.
While the control circuit assembly coordinates the various servomotors which move the weldhead and feed the weld wire to the working end of the tungsten electrode, its most important function for the purposes of this application is to control of the electrical characteristics of the output of the power supply.
Generally speaking, such control circuit assemblies regulate the output of a power supply by maintaining the current of the output at a desired level, while allowing the voltage to vary. To effect such current level control, the output of the power supply includes a shunt resistor across which are the connected the leads of an ammeter which forms part of the control circuit assembly. While the shunt resistor is characterized by a low level of resistance to minimize its interference with the output of the power supply, the voltage drop across it, which is typically only one millivolt per 100 amps, is still readily measurable by the ammeter of the control circuit assembly. In operation, the combination of the shunt resistor and the ammeter form a closed feedback loop which cooperates with various switching transistors in the control circuit assembly to maintain the amperage of the output of the power supply at a level which is set by a thumbwheel switch located in the programmer.
To control the characteristics of the current output, the control circuit assembly further includes a current pulser for pulsing the output current between a relatively low "background" current and a relatively higher "working" current. In a typical AGTAW welding operation, the background current may be 90 amps, while the working current might be 180 amps. The switching frequency between the background and working currents may be adjusted by the programmer to a maximum frequency of about ten cycles per second. The periodic use of a low background current between pulses of a working current maintains the molten puddle created by the electric arc in a semi-solid condition so that molten metal will not flow out of the situs of the weld from gravity if the weld site is tilted or inverted.
Finally, the programmer of a control circuit assembly of a typical AGTAW welder includes an upslope and downslope circuit for linearly ramping the current level of the arc over a time period which is typically between about four and ten seconds. In addition to helping the operator of the welder "get started" in producing the desired weld, the downsloping characteristics of this circuit prevent cracks from occurring in the molten puddle at the termination of the welding operation by shutting off the current in a gradual (as opposed to abrupt) manner.
In contrast to AGTAW welders, AGMAW welders use the weld wire itself as the electrode in the weldhead. Such direct melting of the weld wire itself, in combination with the higher outputs associated with AGMAW power supplies, allows AGMAW welders to deposit far more metal in a shorter period of time than AGTAW welders. While a AGTAW welder may be able to deposit 100 inches of weld wire per minute, a AGMAW welder is capable of depositing up to 1,000 inches of such weld wire per minute. Because most industrial welding operations do not require such quick and large depositions of metal, AGTAW welders are far more common in the industrial marketplace than AGMAW welders. This fact, coupled with the need for higher capacity (and hence more costly) power supplies in AGMAW welders, renders AGMAW welders substantially more expensive than AGTAW welders. Yet there are some welding operations which, in order to be performed in a practical and cost efficient way, necessarily require the high deposition rate which can only be achieved with a AGMAW welder. While combined AGTAW/AGMAW welders are known in the prior art, such welders are at least as costly as AGMAW welders since they incorporate all of the components of an ordinary AGMAW welder. While it has been proposed to avoid the high cost of purchasing commercially available AGMAW welders by modifying an existing AGTAW welder so that it has AGMAW capabilities, thus far there has been no known way to effect such a conversion easily, inexpensively, and reversibly so that the AGTAW capabilities of the welder may be reclaimed. The difficulties of converting a AGTAW welder into a AGMAW welder go far beyond the provision of a higher capacity power supply and a faster wire feed motor. For example, to accommodate the high rate of weld wire deposition, the current pulser of a AGMAW welder must be capable of operating at frequencies of up to 900 cycles per second which is much higher than the 9.9 cycles per second capacity of AGTAW welder current pulsers. Additionally, n order to prevent the weld wire from being frozen in the weld puddle at the termination of the welding operation, a burnback circuit must be provided that burns the weld wire back from the puddle at this juncture.
Clearly there is a need for a welding system which is capable of inexpensively and easily converting readily available and low-cost AGTAW welders into AGMAW welders. Ideally, such a system would allow the converted AGMAW welder to be converted back into a AGTAW welder so that the AGTAW capabilities of the welder are not lost.